This invention relates to mask programmable read-only-memory (ROM). Read only memory is more efficient and denser than random access memory. One use for read-only-memory is for storing a microprocessor's microcode. A mask programmable ROM is a read-only-memory whose contents are programmed during the wafer fabrication process.
A mask programmable ROM process is shown in FIG. 1. FIG. 1 is cross-sectional view of a ROM transistor which was produced with a threshold adjust implant. In this process, two types of transistors are formed to produce a read-only-memory circuit. The first type of ROM transistor remains non-conducting at gate voltages of around 5 to 5.5 volts. A threshold adjust implant step sets the transistor's threshold voltage at 5.5 volts. In the threshold voltage adjust step, boron ions are implanted into the channel 2. This threshold adjust implant ensures that the transistor will be off for gate voltages up to 5.5 volts or above. After the channel 2 is doped with the boron ions, a polysilicon line 4 is formed on top of the channel 2. Phosphorous doped regions 6 and Arsenic doped regions 10 are produced in the silicon substrate to form the source and drain.
The other type of ROM transistor has a normal threshold adjust implant in a manner that these transistors have threshold voltages of around 0.8 volts. These transistors will turn on at gate voltages at or below 5 volts. Typically, the threshold voltage for this transistor is in the range 0.5 volts to 1.0 volts.
This method is used for transistor sizes between 1.2 and 2 microns. A disadvantage with this threshold adjust method is that this method does not work well as the semiconductor circuit scales.
Therefore, an object of the present invention is to provide a method of mask programmable ROM transistor fabrication that works well as the semiconductor circuit scales below 1 .mu.m.